He started to pray then he saw a spot of blood on his thobe; should he interrupt his prayer?

Firstly: 

Blood is najis (impure) according to most of the scholars. See the answer to question no. 114018 . 

Secondly: 

If a person starts to pray and whilst praying he notices a small spot of blood on his thobe, he should complete his prayer and he does not have to exit his prayer in order to purify his garment.

That is because a small amount of impurity is pardoned and it does not have to be washed. 

It says in al-Mughni (1/409): 

If he prays and there is some impurity on his garment, even if it is a little, he should repeat his prayer, unless it is a small amount of blood or pus, that one would not think of as anything significant. 

Most of the scholars think that a small amount of blood or pus is pardoned, because of the report narrated from ‘Aa’ishah (may Allah be pleased with her) who said: “One of us would have a garment that she wore whilst menstruating or when she became junub, then she would see a drop of blood on it, so she would put some saliva on her fingernail and scratch it. According to another version: One of us would have only one garment that she would wear when menstruating, and if any of her blood got on it, she would wet it with her saliva then scratch it with her fingernail. Narrated by Abu Dawood. 

This indicates that it is pardoned, because saliva is not a means of purification and by doing so the impurity would be transferred to the nail. The report indicates that this was something they did frequently. Something like this could not have been hidden from the Prophet (blessings and peace of Allah be upon him) and could not have happened except on his instructions. End quote. 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: A little blood and what may accompany it of pus or matter, and the like, which one would not regard as anything significant, is pardoned."(Sharh al-‘Umdah, 1/103)

The scholars of the Standing Committee were asked (5/363): 

If there is a small amount of impurity, like a drop of blood the size of a grain of pearl millet, do I have to do anything about it? 

They replied: Impurity other than blood, pus or matter is not pardoned, whether it is a large amount or small. As for blood, pus and matter, a small amount of it is pardoned if it came out of anyone other than the front passage, because avoiding small amounts of it is very difficult, and Allah, may He be exalted, says (interpretation of the meaning): 

“and [Allah] has not laid upon you in religion any hardship” [al-Hajj 22:78] 

“Allah intends for you ease, and He does not want to make things difficult for you” [al-Baqarah 2:185].

End quote. 

Shaykh ‘Abd al-‘Azeez ibn ‘Abdullah ibn Baz, Shaykh ‘Abd ar-Razzaaq ‘Afeefi, Shaykh ‘Abdullah ibn Ghadyaan, Shaykh ‘Abdullah ibn Qa ‘ood 

And Allah knows best.